While damages caused by ultraviolet (UV) radiation is often associated with exposure to sunlight and tanning beds, findings of a new study have revealed that UV light can continue to harm the skin and inflict cancer-causing damages hours after exposure and even in the dark.

In a new study published in the journal Science on Thursday, Douglas Brash, from the Department of Therapeutic Radiology at Yale University School of Medicine, and colleagues said that much of the potentially cancer causing effects of UV radiation from tanning beds and sunlight happen up to four hours after exposure because of the chemical changes that involve the pigment melanin.

UV light exposure is known to damage the DNA in the cells known as melanocytes, which produce the melanin responsible for giving skin its color. The damage is often attributed as a major cause of cancer, the most prevalent form of cancer in the U.S.

Experts previously believed that melanin protects the skin by blocking the damaging UV light but evidence from studies suggests that it is also linked with skin cell damage. The new study has found that melanin both has protective and harmful effects.

For their study, the researchers exposed melanocyte cells to UV radiation that caused a type of DNA damage called cyclobutane dimer (CPD) and found that the melanocytes did not only generate CPDs immediately but continued to do so hours after the end of the UV exposure.

The cells without the melanin were also observed to only generate the DNA damage only during the exposure showing that melanin both has carcinogenic and protective effects.

"If you look inside adult skin, melanin does protect against CPDs. It does act as a shield," Brash said. "But it is doing both good and bad things."

The researchers also tested the extent of the damaged after exposure to UV light by preventing the normal DNA repair in mouse melanocyte cells and found that half of the DNA damage were dark CPDs, which are created in the dark.

"These 'dark CPDs' constitute the majority of CPDs and include the cytosine-containing CPDs that initiate UV-signature C→T mutations. Dark CPDs arise when UV-induced reactive oxygen and nitrogen species combine to excite an electron in fragments of the pigment melanin," the researchers wrote.

The researchers said that a sunscreen that protects against this particular damage can be possibly developed.